The present invention relates to foamed products. More particularly, the present invention relates to unique foam products having exceptionally good cushioning properties at low static loadings and low densities. More particularly, the present invention relates to such a foam comprising a plurality of coalesced distinguishable expanded strands or profiles.
Foamed objects comprising a plurality of coalesced distinguishable expanded strands of foamed polymers (strand foams) have been previously disclosed in U.S. Pat. No. 3,573,152. The foam objects are prepared by extruding a foamable thermoplastic material through a multi-orefice die plate, whereby the individual foamable elements of the strand are formed, expanded and coalseced upon emerging from the die orifices. For the teachings contained therein, U.S. Pat. No. 3,573,152 is herein incorporated in its entirety by reference thereto.
Although the previously mentioned U.S. Pat. No. 3,573,152 discloses that polyethylene resins may be appropriately employed in the preparation of strand foams, despite diligent efforts by the present inventors, no polyethylene strand foam has been successfully prepared following the teachings of such reference.
Closed cell polyethylene foams are widely employed in the field of packaging in order to provide cushion properties. Fragile objects intended to be transported may be encased and supported in a closed cell foamed polyethylene cushion adapted to conform to the external shape of the object for which protection is desired. Presently known polyethylene foam cushion materials posses properties adapted to provide particular cushioning performance. For example, higher density foams are suitably employed to achieve peak deceleration forces between about 40 to 50 G's (the gravitational constant) at static loadings of between about 0.5 and 1.5 pounds per square inch in standard 24 inch drop tests. At reduced static loadings, between about 0.1 and about 0.5 pounds per square inch, lower density foams on the order of about 1.2 to about 1.8 pounds per cubic foot may be employed. However, suitable cushioning, i.e. peak deceleration forces less than about 50 G's are not obtainable except upon the use of thicker amounts of cushioning foam. Larger thicknesses of cushioning foam result in excess packaging sizes and concomitant elevated shipping costs.
It would be desirable to provide a closed cell cushioning foam adapted to provide improved cushioning properties at reduced static loadings.
In order to provide such an improved closed cell cushioning foam, the present inventors have investigated preparing strand foam or coalesced foam by the techniques disclosed in U.S. Pat. No. 3,573,152.In particular, in using polyethylene resins and standard chlorofluorocarbon blowing agents and a suitable die having a multitude of closely spaced small holes therein, the emerging strands could not be made to coalesce or adhere together under any processing conditions employed. In particular, when the foaming temperature of the resin was increased in an attempt to provide a tacky surface to the emerging strands, foam collapse was experienced. Similarly, when the strands were reheated after emerging from the die face, foam collapse was again experienced.
It has now been discovered that the ability of a foamable resin formulation to form a coalesced foam structure upon exiting a die containing a multiplicity of orifices and subsequently foaming is dependent on the existence of sufficient adhesion between neighboring foam surfaces at the temperature required for such foaming. In particular, certain resins do not possess sufficient surface tacticity in the melt at the temperatures required for foaming to form the desired coalesced structure.
It would be desirable to provide a method for producing a strand foam comprising a polyolefin resin, particularly a polyethylene resin.
In addition, it would be desirable to provide a polyethylene resin strand foam suitable for cushioning objects having improved cushioning properties at low static loadings.